Through observation, Dr. Montessori found the development of each person must experience 4 special development stages: 0-6 years old infancy; 6-12 years old childhood; 12-18 years old adolescence and 18-24 years old adulthood. Particular development goal exists in each development stage. People's sensitive property in each stage will help them to rapidly and easily realize the goal of this stage in the particular direction. Therefore, only by gathering the children belong to the same stage in the same environment, can educators really and effectively create and operate one development environment in conformity with the common goal and property of this stage. 

In the mixed-age environment, difference of children's natural ages children will contribute to lots of mutual help and learning opportunities instead of mutual competition. Children can find out their own past and future growth track from other children of different ages. For this reason, during formation and development of children's personality, mixed-age environment can provide more multiple and abundant optimum social interaction elements.